This invention relates to a method and apparatus for joining a zirconium alloy appendage to a zirconium alloy member, and more particularly to a method and apparatus for joining a zirconium alloy appendage to a zirconium alloy member by brazing without substantial alpha to beta transformation of the zirconium alloy member.
In the current induction braze process of joining spacers, or other appendages onto the sheaths of nuclear fuel elements composed of zirconium alloy, the heat required to effect a satisfactory joint results in substantial alpha to beta transformation of the surrounding and/or underlying areas of the member to which the appendage is joined. The transformation of the member is difficult to avoid since the alpha to beta transformation temperature of zirconium alloy is lower than the required brazing temperature.
This transformed structure in the sheath limits the performance of the nuclear fuel elements. In addition to structural changes of the zirconium alloy material, heating of the member, to the extent of current practice, may have other adverse effects. For example, such heating of a nuclear fuel sheath may result in distortions that make it difficult to load the sheath with the fuel material, or, if applied to a member already containing the fuel material, may result in adverse reactions in the fuel material thus contained.
Methods of joining whereby the thickness of the transformed zone is reduced have been proposed. For example, in U.S. Pat. No. 3,085,141 to W. Rossner entitled "Hard-Soldering Method, Particularly for Joining Nuclear-Reactor Components," it is proposed that the uncoated member be heated to a temperature above the melting point while the coated member remains unheated. When joined, the temperature of the joined components drops rapidly. This method, involving rapid temperature drop, tends to result in high residual stresses which may well be detrimental to product performance, particularly under cyclic loading conditions. This method may also not result in sufficiently sound joints.